Adeno-associated virus (AAV)-7 and -8 poorly transduce vascular endothelial cells and are sensitive to proteasomal degradation

Adeno-associated virus serotypes: vector toolkit for human gene therapy

Recombinant adeno-associated viral (AAV) vectors have rapidly advanced to the forefront of gene therapy in the past decade. The exponential progress of AAV-based vectors has been made possible by the isolation of several naturally occurring AAV serotypes and over 100 AAV variants from different animal species. These isolates are ideally suited to development into human gene therapy vectors due to their diverse tissue tropisms and potential to evade preexisting neutralizing antibodies against the common human AAV serotype 2. Despite their prolific application in several animal models of disease, the mechanisms underlying selective tropisms of AAV serotypes remain largely unknown. Efforts to understand cell surface receptor usage and intracellular trafficking pathways exploited by AAV continue to provide significant insight into the biology of AAV vectors. Such unique traits are thought to arise from differences in surface topology of the capsids of AAV serotypes and variants. In addition to the aforementioned naturally evolved AAV isolates, several strategies to engineer hybrid AAV serotype vectors have been formulated in recent years. The generation of mosaic or chimeric vectors through the transcapsidation or marker-rescue/domain-swapping approach, respectively, is notable in this regard. More recently, combinatorial strategies for engineering AAV vectors using error-prone PCR, DNA shuffling, and other molecular cloning techniques have been established. The latter library-based approaches can serve as powerful tools in the generation of low-immunogenic and cell/tissue type-specific AAV vectors for gene delivery. This review is focused on recent developments in the isolation of novel AAV serotypes and isolates, their production and purification, diverse tissue tropisms, mechanisms of cellular entry/trafficking, and capsid structure. Strategies for engineering hybrid AAV vectors derived from AAV serotypes and potential implications of the rapidly expanding AAV vector toolkit are discussed.

Vascular bed-targeted in vivo gene delivery using tropism-modified adeno-associated viruses

Virus-mediated gene delivery is restricted by the infectivity profile of the chosen vector. Targeting the vascular endothelium via systemic delivery has been attempted using peptides isolated in vitro (using either phage or vector display) and implicit reliance on target receptor expression in vivo. This has limited application since endothelial cells in vitro and in vivo differ vastly in receptor profiles and because of the existence of complex endothelial "zip codes" in vivo. We therefore tested whether in vivo phage display combined with adeno-associated virus (AAV) capsid modifications would allow in vivo homing to the endothelium residing in defined organs. Extensive in vivo biopanning in rats identified four consensus peptides homing to the lung or brain. Each was incorporated into the VP3 region of the AAV-2 capsid to display the peptide at the virion surface. Peptides that conferred heparan independence were shown to retarget virus to the expected vascular bed in vivo in a preferential manner, determined 28 days post-systemic injection by both virion DNA and transgene expression profiling. Our findings significantly impact the design of viral vectors for targeting individual vascular beds in vivo.